There are three major identifiable steps that follow the mining step in the process of obtaining pure metals and/or minerals from rock or ore. These three steps of the overall metallurgical process are mineral processing, metallurgical extraction and metal purification. Mineral processing itself is broken down into one of two steps: comminution and concentration. Metallurgical extraction can be either o hydrometallurgical or pyrometallurgical while metal purification includes smelting, electrowinning or electrorefining of the product coming from the metallurgical extraction steps.
Gold ores containing high concentrations of cyanide-soluble copper minerals, e.g., chalcocite (Cu.sub.2 S), bornite (FeS.multidot.2Cu.sub.2 S.multidot.CuS), malachite [CuCO.sub.3 .multidot.Cu(OH).sub.2 ], azurite [2CuCO.sub.3 .multidot.Cu(OH).sub.2 ], covellite (CuS) and cuprite (Cu.sub.2 O), have traditionally been difficult to treat economically, because of the high costs associated with cyanide consumption during leaching and cyanide destruction during effluent treatment. High concentrations of copper cyanide in the leach liquor can also cause a variety of metallurgical problems, whether gold is recovered by the Merrill-Crowe process, or by adsorption on activated carbon. Consequently, many gold resources that fall into this category are still lying in the ground, awaiting the development of adequate technology for their treatment.
Much research has gone into this problem over the years, and these efforts have generally focused in one of two directions: to develop other leaching chemistries for gold dissolution, i.e., lixiviants that are more selective than cyanide for gold over copper or to suppress the reaction between copper and cyanide by either physically or chemically masking the copper minerals. Neither approach has met with widespread success. This invention provides an efficient and cost effective process for treating gold ores containing high concentrations of cyanide-soluble copper minerals, as well as high copper bearing ores with negligible gold.
Hydrometallurgical extraction of copper and gold has been carried out previously by leaching the ore first with sulfuric acid to dissolve the copper minerals, followed by neutralization of the residual acid and finally by cyanidation to leach the gold. This approach does not work with the sulfide minerals such as chalcocite and bornite, which leach in cyanide solution but not in acid, is often uneconomic due to high acid, base or cyanide consumption, and prolonged leaching times required for two stages of leaching, and is often impractical because of physical changes brought about in the rock by acid treatment. An alternative approach is to leach with cyanide as described in U.S. Pat. No. 3,463,710. This patent discloses an aqueous alkaline cyanide leaching of copper ores followed by electrolyzing the separated solution to recover only a portion of the copper and recovering cyanide. This process, however, suffers from certain limitations; it utilizes low current density, requires high copper concentrations and leads to the oxidation of cyanide at the anode.
There is a need for a process for treating copper-bearing ores or concentrates containing gold by simultaneously leaching both metals, gold and copper, into an aqueous cyanide solution, recovering copper and gold in metallic form and regenerating cyanide ions in an economically efficient process wherein copper is recovered from dilute solutions at high current densities with minimal cyanide oxidation.